Touch systems are well known in the art and typically include a touch screen having a touch surface on which contacts are made using a pointer in order to generate user input. Pointer contacts with the touch surface are detected and are used to generate corresponding output depending on areas of the contact surface where the contacts are made.
In co-pending U.S. patent application Ser. No. 09/610,481 filed on Jul. 5, 2000 for an invention entitled “Passive Touch System and Method of Detecting User Input”, assigned to the assignee of the present invention, the content of which is incorporated herein by reference, a touch system is disclosed. The touch system includes a touch screen coupled to a master controller and a computer coupled to the master controller. The computer executes one or more application programs and provides display output that is presented on the touch screen. The touch screen, master controller, computer and projector form a closed-loop so that user contacts with the touch screen can be recorded by the computer as writing or drawing or used to control execution of application programs executed by the computer.
The touch screen includes a touch surface in the form of a rectangular planar sheet of material bordered by a rectangular bezel or frame. A two-dimensional digital signal processor (DSP) based CMOS digital camera is mounted adjacent each corner of the touch screen. Each digital camera is aimed at the touch screen so that its field of view encompasses a designated edge of the touch surface. In this way, the entire touch surface is within the fields of view of the digital cameras. The fields of view of the digital camera also overlap so that a pointer in close proximity to the touch surface appears within the fields of view of at least two of the digital cameras. This allows the position of such a pointer relative to the touch surface to be calculated using triangulation.
During operation of the touch system each digital camera acquires images of the touch surface within its field of view at a desired frame rate. The acquired images are processed by the digital cameras to determine if a pointer is in the acquired images. When a pointer is in an acquired image, the acquired image is further processed by the digital camera that acquired the image to determine the median line or tip of the pointer within the acquired image. The median line or tip of the pointer is represented by a number. This pointer information is converted into a pointer information packet (PIP) by the digital camera and the PIP is queued for transmission to the master controller.
The master controller polls the digital cameras for PIPs. When the master controller receives a PIP, the master controller divides the number representing the median line or tip of the pointer by the resolution of the digital camera. The master controller then multiplies this result by field of view (FOV) of the digital camera and then subtracts a fixed error correcting calibration angle δ to yield an angle φ. The calculated angle φ is presumed to be the angle formed between the designated peripheral edge of the touch screen encompassed in the field of view of the digital camera that generated the PIP and a line extending from the optical axis of the digital camera that intersects the pointer within the image.
As mentioned above, the aiming of the digital cameras ensures that when a pointer is brought in close proximity to the touch screen, the pointer is captured in images acquired by at least two digital cameras. As a result when a pointer is brought in close proximity to the touch screen, at least two PIPs are received by the master controller and hence, two angles are calculated by the master controller. With two angles available, the master controller determines the intersection point of the lines extending from the optical axes of the digital cameras which generated the PIPs, that intersect the pointer within the acquired images. Using triangulation the position of the pointer relative to the touch screen in Cartesian coordinates is calculated. The master controller in turn transmits this pointer position data to the personal computer. In this manner, the pointer position data transmitted to the personal computer can be recorded as writing or drawing or can be used to control execution of application programs executed by the computer. The computer also updates the display output so that information presented on the touch surface reflects the pointer activity.
As mentioned above, each digital camera is mounted adjacent a corner of the touch screen and aimed so that its field of view encompasses a designated peripheral edge of the touch surface. Ideally, the extremity of the field of view of each digital camera extends slightly beyond the designated peripheral edge of the touch screen by a known amount so that the angles calculated by the master controller are based on a reference frame that corresponds to the touch screen. However, in reality the fields of view of the digital cameras are angularly offset with respect to the peripheral designated edges of the touch screen by unknown amounts due to mechanical tolerances, optical effects and digital camera placement.
In the touch system described above, during calculation of the angles a fixed error correcting calibration angle δ is subtracted from the calculated angles to take into account the angular offsets of the digital cameras. This calibration of course assumes that the angular offsets of the digital cameras are known and equal. Unfortunately, the angular offset of each digital camera usually differs. Also, the angular offset of each digital camera may change during shipping, installation etc. of the touch system. As a result, the angular offsets of the digital cameras are typically not properly compensated for by the fixed error correcting calibration angle. Unless the actual angular offsets of the digital cameras are known, when the position of a pointer relative to the touch surface is calculated using triangulation based on the calculated angles, the calculated position may be significantly different than its actual position. To complicate matters the calculated position of the pointer may vary significantly depending on the pair of digital cameras whose image data is used to triangulate the position of the pointer. As will be appreciated, this makes it difficult to calculate accurately the position of a pointer relative to the touch screen. In an interactive system where the touch screen is mapped to a computer display so that contacts on the touch screen can be used to invoke computer functions, accurately determining the contact position of the pointer on the touch screen is extremely important.
It is therefore an object of the present invention to provide a novel method of calculating camera offsets to facilitate object position determination using triangulation.